1. Field of the Invention
The present invention relates to a boundary acoustic wave filter used as, for example, a bandpass filter of communication equipment. More specifically, the present invention relates to a boundary acoustic wave filter that utilizes boundary acoustic waves propagating along the boundary between a first medium layer and a second medium layer.
2. Description of the Related Art
Various surface acoustic wave devices have been used as an RF filter and an IF filter for cell phones, a resonator for VCOs, a VIF filter for televisions, and other suitable devices. The surface acoustic wave devices utilize surface acoustic waves such as Rayleigh waves or first leaky waves that propagate on the surface of a medium.
Since surface acoustic waves propagate on the surface of a medium, they are sensitive to changes in the surface condition of the medium. Accordingly, in order to protect the surface of the medium on which surface acoustic waves propagate, a surface acoustic wave element is hermetically sealed in a package including a cavity facing the propagating surface. The use of such a package including a cavity inevitably increases the cost of the surface acoustic wave device. Furthermore, since the dimensions of the package are significantly larger than the dimensions of the surface acoustic wave element, the size of the surface acoustic wave device is inevitably increased.
On the other hand, elastic waves include not only the above surface acoustic waves, but also boundary acoustic waves that propagate along the boundary between solids.
For example, “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrason., VOL. SU-25, No. 6, 1978 IEEE (Non-Patent Document 1) discloses a boundary acoustic wave device in which IDTs are provided on a 126° rotation Y-plate X-propagation LiTaO3 substrate, and a SiO2 film having a predetermined thickness is provided on the IDTs and the LiTaO3 substrate. According to the device, (SV+P)-type boundary acoustic waves, which are referred to as Stoneley waves, propagate. In the description of Non-Patent Document 1, when the thickness of the SiO2 film is about 1.0λ (wherein λ represents the wavelength of the boundary acoustic waves), the electromechanical coefficient is about 2%.
Boundary acoustic waves propagate in a state in which energy is concentrated at a boundary portion between solids. Accordingly, since energy is negligible on the bottom surface of the LiTaO3 substrate and the top surface of the SiO2 film, the characteristics are not changed by changes in the surface condition of the substrate or the thin film. Consequently, a package including a cavity is not necessary, and thus, the size of the elastic wave device is reduced.
It is known that bulk waves propagating through a medium layer include three types of wave, i.e., longitudinal waves, fast transverse waves, and slow transverse waves. These three types of wave are referred to as P waves, SH waves, and SV waves. The waves of the SH waves and the SV waves that become the slow transverse waves depends on the anisotropy of the material. When the material is isotropic, two types of waves, i.e., longitudinal waves and transverse waves are generated.
Among the above-described three types of bulk wave, the slow transverse waves have the lowest sound velocity.
On the other hand, boundary acoustic waves that propagate through an anisotropic material, such as a piezoelectric substrate, propagate with a combination of three partial wave components, i.e., the P waves, the SH waves, and the SV waves. The types of boundary acoustic wave are classified in accordance with a main component. For example, boundary acoustic waves known as Stoneley waves are boundary acoustic waves primarily composed of the P wave component and the SV wave component, and SH-type boundary waves are boundary acoustic waves primarily composed of the SH wave component. Under some conditions, boundary acoustic waves propagate without a combination of the above components.
Boundary acoustic waves normally propagate with a combination of the above three partial wave components. Therefore, for example, in boundary acoustic waves whose sound velocity is higher than that of the SH waves, the SH wave component and the SV wave component leak. In boundary acoustic waves whose sound velocity is higher than that of the SV waves, the SV wave component leaks. These leaked components cause propagation loss of the boundary acoustic waves. Accordingly, it is believed that, in boundary acoustic waves that propagate along the boundary between two medium layers, the sound velocity of the boundary acoustic waves is less than the sound velocity of the slow transverse waves of the two medium layers, thereby concentrating the energy of the boundary acoustic waves near electrodes disposed between the two medium layers to obtain a condition in which the propagation loss is zero.
In radio equipment used in a frequency band such as the 800 MHz band, the 900 MHz band, or the 1,900 MHz band, which is represented by cell phones, transmission and reception are performed at the same time in a transmission band and a reception band that have different frequencies. In such radio equipment, a transmission filter in which the transmission band is the passband and the reception band is the stopband, and a reception filter in which the reception band is the passband and the transmission band is the stopband are used.
Each of the transmission filter and the reception filter may be provided as a single chip component. Alternatively, the transmission filter and the reception filter may be combined to define an antenna duplexer, i.e., duplexer.
Where a boundary acoustic wave device is used as the transmission filter and the reception filter, as described above, even when a boundary acoustic wave filter is prepared under conditions in which the propagation loss is sufficiently reduced, it is difficult to achieve a satisfactory frequency characteristic. Specifically, the present inventors have confirmed the following phenomenon through experiments. Even if the loss is reduced by eliminating a leakage component only in the passband to reduce the propagation loss, when a leakage component is present in the attenuation band disposed at the high-frequency side of the passband, the electric power of the attenuation band is directly transmitted by the leakage component from the input port to the output port, resulting in an insufficient attenuation. Consequently, it is difficult to achieve a satisfactory frequency characteristic. Accordingly, when a boundary acoustic wave filter is used as the transmission filter and the reception filter, particularly in the transmission filter, the attenuation in the stopband disposed at the high-frequency side of the passband, that is, in the passband of the reception filter is not satisfactory.